The present invention relates to a semiconductor light receiving element and an optical transmission device that are used in fields such as an optical communications field.
In recent years, in order to expand and enhance the information service network, the cost lowering has been desired in the optical communications field also. The key to constructing a low cost optical communications system is cost lowering of an optical module for converting an optical signal to an electrical signal and the electrical signal to the optical signal. This requires an optical device that can be integrated on the same substrate by a simple implementing system.
Optical devices, typically a semiconductor laser diode, a light modulator, an optical switch and so on, generally have an edge emitting/incidence type structure and thus receives and emits light in a direction parallel to the substrate surface. On the other hand, semiconductor light receiving elements are generally of surface light receiving type or surface illuminated type that receive a light signal from a direction perpendicular to the substrate surface plane. Accordingly, since surface light receiving type semiconductor light receiving elements differ from other optical devices in the receiving and emitting direction of the light, they are poor in terms of coherence with other devices from viewpoint of integration.
On the other hand, there has been conventionally known an edge emitting/incidence type semiconductor light receiving element that has been disclosed in, for example, JP-A-64-90570 and has a good coherence with other optical devices. However, in the edge emitting/incidence type semiconductor light receiving elements, due to the structure thereof, the positional relationship between an emitting position of the light from an optical device and a light receiving surface of the semiconductor light receiving element depends on a sensitivity of the light receiving element exceedingly. On account of this, in the passive type alignment, means is said to be required that allows a position of the edge emitting/incidence type semiconductor light receiving element to be measured on the optical devices with a high accuracy.
In the above-described prior art, when the edge emitting/incidence type semiconductor light receiving element is mounted on an optical device, markers are formed on the optical device side of the light receiving element and at the optical device part. Then, using light that is capable of transmitting through the both, i.e., for example, light with a wavelength of around 1.3 xcexcm, positions of the respective markers are detected, thereby executing the positioning of the edge emitting/incidence type semiconductor light receiving element on the optical device.
However, in the case of light receiving elements for receiving light of about 1.3 to 1.6 xcexcm that are frequently used in fields such as the optical communications, the transmitted light with the wavelength of around 1.3 xcexcm is absorbed at the light receiving layer. This makes it difficult to detect the positions of the markers that serve as a guide for positioning.
An object of the present invention is to provide a semiconductor light receiving element that facilitates the positioning thereof at the time when the semiconductor light receiving element having an absorbing layer of an arbitrary wavelength is mounted on an optical device, and a fabricating method thereof.
Also, another object of the present invention is to provide a low cost optical module or an optical transmission device on which the above-described semiconductor light receiving element according to the present invention is mounted as a signal receiving element or as a monitoring element of semiconductor laser light.
In order to accomplish the above-identified objects, an optical element according to the present invention includes a light absorbing layer and is to be mounted on a specific position of a certain member by positioning it to the specific position by irradiating the element with light having a wavelength range at least a part of which may be absorbed by the light absorbing layer and detecting a transmitted light, wherein a region which does not include the light absorbing layer and which has a higher transmission rate for the irradiation light than the remaining region so as to be identifiable in terms of its position is provided within a 2-dimensional projected region of the optical element projected onto a plane which is generally parallel to a plane on which the optical element is to be mounted.
Also, in order to accomplish the above-described objects, a light receiving element according to the present invention includes a light absorbing layer and is to be mounted on a substrate which in turn includes another optical device, wherein a part of the light absorbing layer facing a portion of at least the substrate out of the substrate and light receiving element which functions as a positioning marker is removed.
More concretely, as illustrated in, for example, FIG. 1, an edge emitting/incidence type semiconductor light receiving element is provided that has a light absorbing layer 19 sandwiched between an upper 2nd core layer 18 and a lower 2nd core layer 20 and between an upper clad layer 17 and a lower clad layer 21, wherein the edge emitting/incidence type semiconductor light receiving element has a marker detecting space region (a positioning region) 24 for measuring a positioning marker 23 on an optical device on which the edge emitting/incidence type semiconductor light receiving element is mounted. Here, a portion in FIG. 1 at which a signal light 11 and the light absorbing layer 19 are in contact with each other is a light receiving region of the edge emitting/incidence type semiconductor light receiving element.
In the above-described edge emitting/incidence type light receiving element according to the present invention, as illustrated in, for example, FIG. 4, when the light absorbing layer 19 is formed, the crystal layers are selectively grown using a mask or the like so that a light absorbing layer 19 in the light receiving region remains present and a portion of the light absorbing layer 19 facing the positioning marker 23 on the optical device is vacated (not formed), thereby forming the space region 24 for detecting the marker 23. The light transmission rate of the illumination light (hereinafter, referred to as detection light), which is radiated and transmits through the marker detecting space region 24 so as to detect the positioning marker 23, is configured to exceed, for example, 30% of the detection light with which the edge emitting/incidence type light receiving element is irradiated.
According to the above-described edge emitting/incidence type light receiving element of the present invention, attenuation due to the light absorbing layer 19 of the positioning detection light radiated from below is extremely small as compared with the case where the light absorbing layer 19 remains present. This makes it easier to observe the marker 23 through the light receiving element, despite that the marker 23 on the optical device is shadowed by the edge emitting/incidence type light receiving element, thereby enhancing the production efficiency.
Also, according to the above-described edge emitting/incidence type light receiving element of the present invention, since mounting accuracy on an optical module substrate 26 is also enhanced, the light receiving efficiency is enhanced as well. By virtue of this, the edge emitting/incidence type light receiving element according to the invention may be configured such that an edge emitting/incidence type light receiving element 31 according to the invention is integrated on a substrate 37 having a V shaped groove for securing an optical fiber 38, without optical lenses, so as to be used as a signal receiving element or as a monitoring element for a semiconductor laser diode 41, as shown in FIGS. 7 and 8.
Also, as shown in FIG. 9, by using a passive alignment method, a signal-receiving edge emitting/incidence type light receiving element 55 according to the present invention may be integrated on a V-shaped groove substrate 54 having an optical fiber 52 for an incident light signal. Moreover, a preamplifier IC 56 may also be mounted and, in addition, they may be packaged with a base 53 and a cap 51 made of ceramic or resin. Instead of using the base 53 and the cap 51, a resin transfer molding may be employed.
Also, an edge emitting/incidence type light receiving element according to the invention may be used in a light sending module in which the light receiving element is used for monitoring a semiconductor laser or edge emitting/incidence type light receiving elements according to the invention may be used in a light sending/receiving module in which one of the elements used for reception and the other used for monitoring a semiconductor laser are respectively mounted on a same substrate.
Furthermore, as illustrated in FIG. 10, an optical module 64 in which an edge emitting/incidence type light receiving element according to the present invention is mounted and is equipped with an optical fiber 63, and an electronic circuit such as a signal-receiving IC 61, may be mounted on the same board 62 so as be used as a sending, a receiving, or a sending/receiving apparatus in an optical transmission system.
Also, in order to accomplish the above-described objects, a method of fabricating an edge emitting/incidence type semiconductor light receiving element according to the invention comprises the steps of sequentially laminating a plurality of different thin film layers including a light absorbing layer on a substrate, wherein steps of laminating the light absorbing layer and subsequent layers include either prohibiting thin films involved from growing at a predetermined region so as to allow the semiconductor light receiving element to be adapted to be positioned in place or an etching step to remove a portion of the light absorbing layer present under a predetermined region so as to allow the semiconductor light receiving element to be adapted to be positioned in place.